If there are any die-hard SERVO readers out there, you may recall the four part article back in 2011 about the “Big Walker.” Well, a great deal has changed on this biped since then. Only about 10% of the original parts
remain. Most of those are the servos and they are about to
be replaced, too. There are a few pieces of angle
aluminum, controller board, and the load cells. Pretty much,
that’s it.

Similar to the efforts of the famous self-educated
clockmaker John Harrison, the big biped project is an
attempt to build a precision mechanism that differs from
the standard accepted designs. Novelty and imagination are
the key ingredients. Solving problems is a reiterative process
of analysis and rework. This type of project takes years of
hard work. Disappointment is common.

As we get closer to our goal of a stable system that
meets our constraints, we are thankful that we did not
merely go down the path of the standard accepted design
of square flat feet. We will leapfrog the other heavy footed
designs and obtain a working biped that mimics nature in
efficiency and elegance.

This is a very complex project requiring several
engineering disciplines. While we are both professional
software and firmware guys, it has been a learning
experience in mechanical design and fabrication. We are
learning mostly by trial and error that what works on paper
doesn’t always work in practice. This has prompted us to
rebuild the biped a third time.

We discovered that the second design constructed two
years ago would not work. Here is why.

In order to walk well, a biped must be able to support
itself on one leg. The first two designs could not. It was
initially presumed that if most of the weight could be
translated to one side, that the opposite leg could be
moved into a more forward position to catch the falling
mass. This is a dynamic gait similar to the way humans
walk. Like a pendulum, the mass would swing to one side
and then back.

As humans, we throw our weight outside our center of
mass and fall towards a determined spot where we place
our foot to either stop our movement or transfer our mass
to a new vector. This works in theory but when put into
action, things get more difficult. We lost control of the
system due to low power, slow reaction time, and lack of
stiffness. We had trouble just with the one leg stance. Each
attempt of moving from a two-legged stance to one side
yielded different results. Our rigid body dynamics analysis
proved incorrect. The truth is “nothing is rigid.”
Power is crucial for holding the given mass of the biped
from collapsing. Servos are always rated higher than their
actual stall torque. The servo’s rated at 400 oz-in of torque
turned out to provide about one half that in a pure stall.
The movement from two legs to one caused the power
consumption to spike. The servos got hot and soon went
into current-limit protection mode; they shut down and the
biped collapsed.

The pulley reduction designed in the first system does
protect the servos from damage and creates more torque,
but even a 2:1 increase in torque was not enough.

It was presumed that a fast reaction time to the
moving mass could keep the biped in positions where the
stress on individual servos was minimized. This helps, but
cannot replace needed power. It takes many mSecs of time
to change the direction of a servo. It has rotational mass
that may need to reverse.

This isn’t helped by the very slow 50 Hz control time of

by Daniel Albert and Chris Mayer

56 SERVO 10.2013

The Road
to the DARPA
Robotics
Challenge

Go to www.servomagazine.com/index.php?/magazine/
article/october2013_Albert to comment on this article. Part 2:
Mechanical Design